Interpretive Summary: Host immunity is an important influence in parasitic infection, and evasion of immunity is crucial to survival of parasites requiring prolonged association with their hosts. In gastropods, the principal reaction to tissue invasion by metazoan parasites is an aggregation of hemocytes that serves to encapsulate then kill the pathogen in resistant hosts. In contrast, nematodes that infect gastropods are readily encapsulated by hemocytes but survive. This suggests that nematodes employ different strategies to evade immunity in their intermediate hosts. We sought to investigate nematode molecules that function at the host-parasite interface during infection with P. tenuis, a neurotropic nematode parasite that in atypical hosts such as sheep, goats and other small ruminants, causes debilitating neurologic disease. From a library of genes, a unique cathepsin B like protease molecule was identified that is developmentally regulated, restricted to larval stages in the intermediate host, and is synthesized coincident with the escape of the infectious organism from the intermediate host. These data are important in that we also showed emerged larvae can survive outside the host for prolonged periods thereby providing a novel avenue of transmission to the definitive host. If involved, the synthesis of the cathepsin B like molecules may be targets for drug or vaccine intervention in other nematode species.

Technical Abstract:
Parelaphostrongylus tenuis is a parasitic nematode that causes debilitating neurologic disease in domestic livestock. We produced a PCR-based cDNA library from infective larvae (L3) to identify molecules that mediate parasitism. An amplicon encoding a homologue of cathepsin B cysteine proteases was identified that incorporated a C29G substitution in the putative active site. Antibodies generated against a recombinant form detected the native protein (Pt-CPR-1) in Western blots of L3, but not adult worms. Immunohistochemistry revealed Pt-CPR-1 was restricted to larval stages within the snail intermediate host, beginning as early as 2 days post-infection (dpi). The protein was present in the intestine and luminal contents, but decreased over time. Studies showed that larvae induced an immune response in snails beginning 1 dpi. Layers of hemocytes encapsulated the larvae immediately after infection and granuloma-like structures formed in chronic infections. Loss of Pt-CPR-1 from L3 and its accumulation in host tissues coincided with degeneration of the granuloma 90-105 dpi when fully-developed L3 emerged from the snail. Our data implicate Pt-CPR-1 in larval development and in emergence of P. tenuis from the snail. Because emerged L3 survived desiccation and cold stress, data suggest that molecules promoting emergence could facilitate transmission to definitive hosts.